Longwei 3060 PS Dead

[metrologist]

Hey all, could really use this.

Measured 120vac to the board at upper right. Looks like the LYT90-1ZT black module switches from a 12v control, but no ac is getting to the xformer at top center. What should I look for?

[Jwillis]

The  LYT90-1ZT is a relay. Does it sound like it's activating? Should make a "click" sound. Not  the same brand but same non the less. https://www.cuidevices.com/product/resource/pr24.pdf You want to look at  Form C for pin connection.

[metrologist]

The ps acts like it is not plugged in. I have not been able to determine how the 12v coil voltage is generated. You can see two small traces leaving the left side of relay, but the rest of the circuit seems isolated from the mains into the board.

The foil side

Looking at bottom right, the large soldered trace is neutral, the shorter one above is line. I think the large rectangle trace to the left of line is the switched side of the relay, and it is hot. The two small traces above that is what I think is coil connection, but no dc between them.

[Jwillis]

You won't see 12V across the coil. Only the voltage drop, which will be near 0V because the coil have a very low resistance. If your getting Live across the relay then the relay is working.
The problem is farther in the circuit.
I can't find a schematic for this power supply which makes it difficult to precisely diagnose problems.

[Jwillis]

Just a Thought. Is the switch for inside/outside sense in the right position?

[teatime]

Please see attached schematic for you reference

[Jwillis]

Please see attached schematic for you reference

Thanks for the help. But unfortunately that schematic is for a linear supply. The  Longwei 3060 is a switching supply. Appreciate it any way.
Your probably referring to the other Longwei in post https://www.eevblog.com/forum/repair/longwei-power-supply-mod-tpr3005-2d/

[metrologist]

I have not been able to find much info on this circuit either. Regarding sense, there does not appear to be external sense terminals. It's not like there is just no output, but no display or anything to indicate it's even plugged in. I did not smell or see anything that looks burned.

In the first image, there is a large transformer top center and I do not measure voltage on its primary. 

[Jwillis]

I have not been able to find much info on this circuit either. Regarding sense, there does not appear to be external sense terminals. It's not like there is just no output, but no display or anything to indicate it's even plugged in. I did not smell or see anything that looks burned.

In the first image, there is a large transformer top center and I do not measure voltage on its primary. 

I wasn't sure which version you had. The new programmable or the older dial type.


 Because I can't see what's under the heat sink, I'm guessing at the circuit based on the trace and possible component placement and function of each.
The first part from the relay is for EMI filtering with the common mode choke and various capacitors. The rectifier Is a guess based on the number of pins and orientation with the screw. The Mosfets I also guessed on based on number of pins and orientation with the screws. If those are not getting a pulsed signal to the gate they will not turn on. Those would feed the large transformer with a square wave. Which would explain why your not getting a voltage there. This would indicate that one of the Mosfet driver chips isn't active.
 

[floobydust]

A little 12V SMPS powers the panel meters and fan, as well as the main SMPS control and relays.
I would look at the two subsections - the 12V aux switcher and the soft-start circuit.

Careful the HV DC is dangerous to poke around and measure. Also these low cost chinese products do not always have decent bleeder resistors so the caps can hold an unexpected charge and bite.

See if there is any HV DCV showing up at the big primary caps. If not then the soft-start circuit may have failed, the big black NTC for inrush-limiting next to the relay I would check its resistance. It's purpose is to slow charge the primary caps and then the relay pulls in to bypass it. Guessing around 10-20 ohms cold.

The 12V supply looks like the DIP-8 and TO-220 mosfet? It could have HV DC present but is not starting up. Some voltage measurements around it or at the IC if you can do this carefully will tell a lot. What is the part number for it.

[metrologist]

The large caps are getting charged. They are in series. It's hard to see, just above the ceramic power resistor on foil side and to the right of that may be two large bleeder resistors. I only measured xformer for dmm ac volts.

Where is the 12v circuit, or should I probe the mosfets noted above?

[floobydust]

Do you see the DIP-8 IC, near the relay on the main board. That I believe is for the 12V SMPS. Is there a p/n on it?
The big mosfets I would leave alone, although it's always a good check for D-S shorts (after big caps are fully discharged, careful). Need the 12V up and running first I think.

[metrologist]

The fets measure 0.468.

IC looks a uc3842b, it has 12.5 vcc, no dc output, vref 0.68, feedback 0.2 - I guess the output is a pwm signal, so I’d need my scope to see if there is output.

https://www.onsemi.com/pdf/datasheet/uc3842b-d.pdf

I cannot read the TO-220

[floobydust]

UC3842 startup voltage is around 16VDC (pin 7) so I'm wondering if it has enough voltage to start. Vref (pin_8) should be 5V so the IC might be dead.
Check if resistor R9 by the TO-220 mosfet is in-spec, it's the one with one end up, and look for burnt parts there.
If R9 is in spec, I would check the electrolytic capacitor next to the mosfet, and also see if the mosfet is not shorted. I think the sense-resistor is the fuse here for this subsection.

[metrologist]

I measured the fet, outside pins are 0.691, inside are 0.559 and 2.8 or OL. Opposite polarity is 2.8 or OL.

I cannot get a reading on cap in circuit, should be 47uf if I read it correctly. Resistor looks and reads like 110k.

I tried to scope out some pins on and around uc3842, with ground to chassis, but first verified 0 ohms to neg output and no ac on dc output terminals. I probed -180V, +/-180 on large caps.

Then I saw the fan kicked a turn, decimals on the displays. I disconnected and cycled power and saw 666 on the display. 

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[floobydust]

It just started up after a long wait? Not uncommon for a bad electrolytic to warm up and work a bit.
Or did you bump a poor connection, touch a node etc.

Look for local witch doctor or voodoo practitioner, it might have a 666 demon in it lol.

[metrologist]

My post wasn't clear as it could be. Probing all IC pins showed -180V referenced to chassis. Is that abnormal?

I'd not be surprised to see +/-180V on the big caps. What I wanted to see was the output of the IC, which I expect to be PWM. I think to see that, I'd need an isolated scope and reference the IC ground pin. But I have not forgotten the Vref being low.

I could shotgun that 47uF cap and see what happens. And then there are other options since the front panel labeling is telling me this is actually a possessed 10A supply...